JPH0834314B2 - Superlattice device - Google Patents

Description

FIELD OF THE INVENTION The present invention relates generally to superlattice devices, and more particularly to devices having layers grown on a surface offset from the main crystallographic plane and having horizontal and vertical periods.

BACKGROUND OF THE INVENTION Recent improvements in semiconductor device technology have been made in several areas. For example, efforts have been directed towards making higher quality silicon, that is, conventionally obtained silicon with fewer defects or less unwanted impurities. In addition, semiconductors other than silicon, such as III-V compound semiconductors such as GaAs, have been developed for many devices. This is because they have higher carrier mobility than in silicon, resulting in faster device characteristics than similar silicon. In addition, many types of devices such as field effect transistors, avalanche photodetectors and double heterostructure lasers have been fabricated and their properties have been investigated.

The development of advanced epitaxial growth techniques such as molecular beam epitaxy has favored the fabrication of many of these devices. These techniques have also enabled periodic structures with unit cells that differ from the constituent semiconductors to be fabricated. Perhaps the first such structure was proposed by Esaki and Tsu, who now proposed a structure commonly referred to as a superlattice. It has a periodic composition change in a direction parallel to the growth direction, ie perpendicular to the major surface of the substrate. It may be called a "vertical" superlattice because its period lies in the direction perpendicular to the main surface of the substrate. For example, IBM Journal of Research and Development (IBM Journal
of Research and Development, 14, 61-65, January 1970. Such structures have alternating layers of different composition or different doping concentrations. For example, having alternating GaAs and AlGaAs layers. The proposed structure has interesting and important device applications. For example, Esaki et al. Predicted that negative differential conductivity would be observed.

Due to the pioneering work done by Eseki, many other periodic structures are also mentioned. Superlattices have been fabricated, for example, with layers having a wide range of thicknesses. If the layers are thin enough, quantization of carrier energy levels becomes important and quantum mechanical effects become apparent. For example, Dingle and Henry are US Pat. No. 3,983, issued on Sep. 21, 1976.
In 2,207, we describe a low current threshold double heterostructure laser whose active region consists of a superlattice. The specifically mentioned superlattice structure has multiple thin wide bandgap layers sandwiched by multiple thin narrow bandgap layers, and due to the increased density of states at low energies caused by quantum effects, Oscillation occurs at a low current threshold.

Other superlattice structures are also mentioned. For example, July 31, 1979 on Dingle, Gossard and Theme.
U.S. Pat. No. 4,163,237, issued to date, describes a modulation-doped device. In one embodiment, the device has a doped broad bandgap layer that is adjacent to a nominally undoped, intrinsically conductive narrow bandgap layer. The conduction band edge of the narrow bandgap layer is
It is energetically down, so carriers from the dopant atoms migrate into the narrow bandgap region. This is desirable for device operation. This is because the carriers are nominally confined in the undoped narrow bandgap region, while the impurity dopant atoms are in the wide bandgap region. Since there is no carrier scattered from impurity atoms,
High carrier mobility occurs.

Still other periodic structures are mentioned. For example, 198 for Dingle, Gossard, Petroff and Wiegmann.
U.S. Pat. No. 4,205,329, issued May 27, 009, discloses a superlattice structure consisting of alternating monolayers of superlattices, i.e. the first plurality of monolayers comprises a first composition, for example GaAs. And they describe a structure sandwiched between a second plurality of monatomic layers having a second composition, eg AlAs.
Of course, an embodiment is also described in which several monoatomic layers of one composition are grown and sandwiched by one or more monoatomic layers of the second composition. Another example is the use of multiple Al
It has a GaAs layer and a Ge layer. The Ge layer is formed in a columnar structure. The notation considered to describe those monatomic superlattices is (A) m (B) n, where A and B represent different semiconductors or the same semiconductor with different doping concentrations, where m and n are monads, respectively. Represents the number of atomic layers A and B. Therefore, the structure is m atomic layer of A, followed by n of B.
It consists of atomic layers.

During the deposition, the growth region of each layer is distinguished, so that the growth of a monoatomic layer superlattice is possible. Above the critical substrate temperature, the main growth region is characterized by atomically rough interfaces.
However, at a substrate temperature in the range below this region, a thin plate-like growth mode is realized, and an atomically smooth interface having a steepness equivalent to one atomic layer is generated. At a deposition temperature below this range, mixed growth of flaky growth and island growth occurs, and a rough interface occurs between, for example, AlAs and GaAs epitaxial thin films. The rough interface is the growth of nuclei on the flat top surface,
It is believed to result from competition between nucleation at the step edge. In other words, the rough interface occurs because part of the layer grows on the already deposited material, ie the monatomic equivalent material is deposited but the coating is not uniform. Island nucleation is promoted by the presence of impurities on the flat top surface. This is advantageous over the layer growth mechanism in a wide temperature range of growth, unlike islands.

SUMMARY OF THE INVENTION A device is described having a superlattice grown on a surface consisting of multiple regions with horizontal periods and having multiple step edges. By "horizontal period" is meant that the period is in a direction essentially parallel to the substrate surface. The superlattice may be described as consisting of (A) m (B) n. Here, A and B are the compositions of the first and second regions, respectively, and m and n are a part or an integer of the monoatomic layer formed by A and B, respectively. The step edges, which are desired to form a periodic array, are conveniently formed by offsetting the surface with respect to the main crystallographic plane. In one embodiment, the superlattice region comprises alternating regions of two semiconductors such as GaAs and AlAs. That is, the superlattice consists of (GaAs) m (AlAs) n, where m and n are the proportions of the edges covered by the region. In other words, it is the number of monolayers of GaAs and AlAs, respectively. One or both of m or n may be less than or equal to 1.0. Additionally, both m and n may be greater than 1.0. In another embodiment, m or n is less than or equal to 1.0,
m + n is greater than 1.0. In a specific embodiment, the surface is offset (100) GaAs substrate.

Description of an Embodiment A cross-sectional view of one embodiment of a device according to the present invention is depicted in FIG. The structure comprises a substrate 1 having a plurality of steps 3 having a height d width w. A first plurality of regions 5 having a first composition and a second plurality of regions 7 having a second composition are arranged on the substrate 1. The first and second regions are sandwiched between each other. The first and second regions on the substrate form a horizontal superlattice. A horizontal superlattice refers to a structure in which first and second regions are alternately arranged in a direction substantially parallel to the substrate surface. The steps form a periodic array. That is, the steps are arranged periodically.

The region typically comprises a semiconductor such as a III-V compound semiconductor. However, other semiconductors and materials that epitaxially grow on offset substrates may be used. More generally, the first and second regions are (A) m
And (B) n, where A and B are the first
And in the second composition, m and n are the numbers of A and B monoatomic layers, respectively. The term "composition" is used to describe not only the major components of the region, but also the dopant. For example, regions 5 and 7 are composed of GaAs and may have different conductivity types.

It is convenient to form the periodic step by shifting the direction of the substrate with respect to the main crystal plane. This is conveniently formed by cutting a misaligned wafer and etching to obtain a stepped surface having a step height equal to the atomic spacing. In practice, it is desirable to grow an unillustrated buffer layer of the same material as the substrate material in order to eliminate unwanted deformations in the surface caused by etching. The resulting misalignment angle, ie the angle between the direction of the superlattice and the main crystal plane, is designated as α.

The two cycles are the m monolayer of A and the n monolayer of B,
It may be formed by alternately depositing. Here, m and n are fractions or integers greater than or less than 1.0. There may be a horizontal period and a vertical period.
Thus, the new type of superlattice is a unit cell vector ▲
It is caused and defined by ▼ and ▲ ▼. Vector is ▲
▼ = (m + n) d / tanα and ▲ ▼ = d / sinα. Where d is the step height, m and n
Is the width ratio of the steps covered by the first and second regions, respectively, and α is the offset angle. For a (100) GaAs substrate, d = 0.283 nm. The percentage of covered step width is equal to the percentage of monolayer formed. Both m and n may be greater than 1.0. That is, the region may be one monoatomic layer or more.

In a specific embodiment, the substrate is made of GaAs and the first
And the second plurality of regions are (GaAs) 0.75 and (Al), respectively.
As 1.25. That is, m and n are 0.
Equal to 75 and 1.25. The (100) substrate had a shift of an angle α with respect to one of the <110> directions.

The superlattice of the present invention is conveniently manufactured by molecular beam epitaxy (MBE). The substrate is cut to the desired angle and polished. This angle is chosen to select the desired step width, typically 1 to 5 degrees,
Larger and smaller corners may be used. However, for (100) GaAs, the misalignment angle within this range is 10
Yields step widths of ˜50 nm. Values in this range are desirable if quantum effects are desired and to avoid unwanted carrier tunneling. The step height is equal to the atomic spacing and is therefore not a function of the offset angle. However, the step spacing or step width increases as the misalignment angle increases.

A standard surface pretreatment was used prior to epitaxial growth. For GaAs and AlAs compositions, epitaxial growth may be performed in a standard high vacuum molecular beam epitaxial growth chamber at temperatures ranging from about 510 ° C to about 610 ° C.
However, this is an approximate range. The precise range is a function of total arsenic pressure, As ₂ / As ₄ ratio and growth rate. The determination of the optimum temperature range can be easily performed by those skilled in the material system.

The quantities of the first and second regions, ie the values of m and n, are precisely controlled, for example by using an automatically controlled shutter system which alternately blocks the Ga and Al beams. By adjusting the shutter system or the time the beam can reach the substrate, the desired values of m and n to be obtained can be obtained. The values of m and n can be integers or fractions and can be greater or less than 1.0. The deposited atoms move to the step edge, and the stepped surface retains a stepped surface during growth, which acts as a nucleus position. Growth is dominated by step-edge nucleation and lateral layer growth.

Fabrication of structures using other semiconductors and materials will be readily accomplished by those of ordinary skill in the art in the discussion above.

The presence of thermal kinks along the step edges can interfere with superlattice fabrication. This is because the probability of nucleation at the kink position is higher, and thus the kink tends to dominate the lateral layer growth. The term "kink" means that there is no step edge uniformity.
Lateral layer growth on different steps along the step edge thus becomes non-uniform on an atomic scale and the superlattice becomes (m +
It becomes irregular for very small values of n). However, better control can be achieved by using higher substrate temperatures during growth and proper choice of step edge orientation.

Many other superlattice structures can be made in accordance with the present invention.
One such structure is depicted in FIG. 2, where the numbers are the same as those used in FIG. 1 to indicate the same elements. In this example, the superlattice region has a composition (Ga
As) 0.5 (AlAs) 0.5. That is, the first and second
Regions are composed of (GaAs) 0.5 and (AlAs) 0.5, respectively, and both m and n are equal to 0.5. A superlattice having a period λ = d / tanα has a period perpendicular to the growth surface, that is, parallel to the main surface of the substrate. It is easily recognized that the first and second plurality of regions form a vertical stack of monatomic layers. That is, regions having the same composition sequentially overlap each other in the monoatomic layer. However, the formation of such a stack can only be obtained in a relatively narrow range of (m + n), as shown in FIG. In Fig. 3 (m + n)
Is plotted horizontally with respect to the angle β between the orientation of the vertically plotted horizontal superlattice and the substrate. The solid line is
In the case of an angle α of 2.5 degrees, the chain line represents the case of an angle of 5 degrees. For the drawn (m + n) and other values of α,
At least one of the plurality of regions overlaps the second plurality of regions and does not overlap itself.

FIG. 4 shows another half-atomic layer superlattice according to the present invention. In this superlattice structure, the composition of the region is (GaAs) 0.
5 (AlAs) is 0.25. That is, A and B are G
For aAs and AlAs, m and n are equal to 0.5 and 0.25, respectively. The angle β is also drawn. The GaAs region forms a vertical stack, but the AlAs region overlaps the GaAs region and does not form a stack. Yet another structure can be easily fabricated by alternating horizontal superlattices with a given value of (m + n) with another horizontal superlattice with another value of (m + n). In addition, the values of m and n may be kept constant and the first and second compositions may be varied. That is,
A second horizontal superlattice (C) o arranged on the first superlattice
(D) p may be available. The same idea applies to the values of C, D, o and p as well as to the values of A, B, m and n. Horizontal superlattices may be sandwiched between vertical superlattices. This allows structures such as quantum well wires to be easily fabricated without the need for lithographic processes. Other variations are easily conceivable.

One such structure is depicted in FIG. The structure consists of multiple stacked (GaAs) 0.5 (AlAs) 0.5 horizontal superlattices sandwiched between stacked vertical (Ga0.7Al0.3As) 100 structures 10 with a period perpendicular to the stepped substrate surface. Alternatively, structure 10 may consist of a (AlAs) 100 monoatomic layer. Typically, as in structure 10, (GaAs) 0.5 (AlA
s) There are about 100 monolayers of 0.5 horizontal superlattice. Fewer numbers are drawn for clarity. Therefore, (Ga
The 0.5 area is surrounded by a larger forbidden material.
Also typically, there is a buffer layer of wide bandgap material (not shown) between the bottom (GaAs) 0.5 monoatomic layer and the substrate. Electrical contacts 20 and 22 to the substrate and top layer
Allows the structure to be used, for example, as a negative resistance device, and when the device is used as a laser,
Allows injection of carriers.

If at least one of the regions surrounding the (GaAs) 0.5 region is doped, for example n-type, interesting device applications arise. Electrons are two-dimensionally confined in the (GaAs) 0.5 region and have the only freedom of translation. Thus, if the horizontal and vertical dimensions of region 7 are small enough that the quantum effect is sufficient, then it is a quantum well wire. More generally, the electron energy level in the doped region should be lower than the conduction band energy level in the adjacent region. Similar considerations apply to p-type conduction.

Other device applications are readily envisioned. For example, a periodic change in composition causes a periodic change in refractive index,
Let itself be a distributed feedback device. In this embodiment, the structure 10 comprises a narrow bandgap material such as GaAs and is surrounded by two wide bandgap regions of opposite conductivity type and suitable period, eg (Ga0.7Al0.3As) and (AlAs) regions. . Therefore, structure 10 is the active layer. The structure is also useful as an electronic single light modulation or optical bistable device. Certain device applications will use relatively strong exciton binding energies. The structure is also useful as a one-dimensional (1D) FET (Field Effect Transistor) as depicted in FIG. The device includes a superlattice region 50 as depicted in FIG. It has a source 52, a drain 54 and a gate electrode 56, which control the current through the superlattice region. The one-dimensional channel runs from the source region to the drain region.

Other device applications for the other superlattice structure described above are readily conceivable as envisioned for the structure described above. For example, the structure depicted in Figure 2 is useful as a diffraction grating for radiation. As is well known to those skilled in the art, compositional variations in the AlGaAs system cause changes in the refractive index.
Visible or near-infrared radiation typically sees only every 10th perturbation. This is because the step width is typically large compared to the wavelengths of radiation in the visible and near infrared regions. However, in the X-ray part of the electromagnetic spectrum, the radiation has a wavelength comparable to the step width. That is, λ is about the same as the step width, and the structure can be easily used as a filter for a broadband X-ray source. The structure depicted in Figure 1 can also function as a diffraction grating. Moreover, this structure also
It can also function as an optical waveguide with light emission occurring in or through the GaAs monoatomic layer. The structure depicted in FIG. 5 also has the following structure 10 when surrounded by a region of low refractive index:
Acts as a waveguide. The waveguide has many steps along its wall, but does not scatter a large amount of light due to the small size of the steps. That is, the step size is significantly smaller than the wavelength of the radiation in the visible or near infrared region. This depicted structure is actually a double heterostructure waveguide, which, when properly pumped, as described above,
Acting as a laser will be readily appreciated by those skilled in the art.

Other depay applications will be readily envisioned. Although the present invention has been specifically described with respect to a horizontal superlattice having two regions sandwiched therebetween, it is understood that the horizontal superlattice may consist of three or more regions having different compositions. It will be easily recognized by the trader.

Quantum well wire lasers emitting at two or more wavelengths can also be fabricated. One such laser that emits at two wavelengths is depicted in FIG. The electrodes are depicted in FIG. 8 for clarity. The structure is GaAs substrate 1, first cladding layer 71 (n-type Ga0.7Al0.3As), first active layer 73, second cladding layer 79 (n-type Ga0.7Al0.3As), second active layer 81. And a third active layer (n-type Ga0.7Al0.3As). The active layer is typically 20-40 nm thick. The first active layer consists of regions 75 and 77, which are p-type (GaAs) 0.5 and n-type (Ga0.7Al0.3As) 0.5, respectively. The cladding layer is typically 100 monatomic layers thick. The second active layer includes regions 83 and 85, which are p-type (GaAs) 0.6 and n-type (Ga0.7Al0.3As) 0.4, respectively.
Is. Regions 75 and 83 are of opposite conductivity type to regions 77 and 85, ie, p and n type, respectively. The width of the GaAs confinement region, ie the width of the energy well, is different in the two active layers and thus the wavelength of the light emitted from the two active layers. Alternatively, the well widths can be the same and Al can be added to the GaAs well in one active layer to change the well width. It should be recognized that the radiation is emitted in a direction perpendicular to the plane of FIG. Implementing this multi-wavelength laser in many material systems will be readily apparent to those skilled in the art.

The manner in which electrical contact is made to the device of FIG. 7 is depicted in FIG. Depicted are substrate 1, superlattice region 181, and third cladding layer 87. Region 181 includes intermediate cladding layers such as layers 71, 73, 79 and 81 shown in FIG. Electrodes 183 and 1 are provided in the n-type and p-type regions, respectively.
85 contacts. The electrodes are conveniently formed by diffusing a suitable dopant and alloying. The emitted radiation is marked λ ₁ and λ ₂ . Of course, the active layers can also have the same composition and dimensions and emit radiation at the same wavelength.

[Brief description of drawings]

1 is a cross-sectional view of one embodiment of the device according to the present invention; FIG. 2 is a cross-sectional view of another embodiment of the present invention; FIG. 3 is a mono-atomic layer of the horizontal axis with respect to the offset angle of two substrates. FIG. 4 is a diagram in which an angle of deviation is plotted perpendicularly to the sum of FIG. 4; FIG. 4 is a sectional view of yet another embodiment of the present invention; FIG. 5 is a sectional view of yet another embodiment of the present invention; FIG. 7 is a sectional view of another embodiment of the present invention; FIG. 7 is a diagram showing a one-dimensional quantum well wire laser; FIG. 8 is a diagram showing electrodes to the laser of FIG. [Explanation of Signs of Main Parts] Multiple Steps ... 3, Height ... d Width ... w, First Area ... 5 Second Area ... 7

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Claims (23)

[Claims] 1. In a superlattice device, a first plurality of regions on a portion of said step on a substrate surface comprising a step having a height and a width has a first composition and another of said steps. A second plurality of regions on the portion has a second composition and the first and second plurality of regions are alternately sandwiched on the surface and are periodic in a horizontal direction parallel to the substrate surface. A superlattice device characterized by being arranged in a regular manner. 2. A superlattice device according to claim 1, wherein the surface of the substrate is displaced from the main crystal plane. 3. The device of claim 2, wherein the first and second plurality of regions form a first horizontal superlattice. 4. The device of claim 3, wherein the first and second plurality of regions comprises (A) n and (B) m, respectively, where n and m are first and second, respectively. A superlattice device characterized in that it is a ratio of said widths covered by a second composition. 5. A superlattice device according to claim 4, wherein A and B are semiconductors. 6. A device according to claim 5, wherein the semiconductor is selected from the group consisting of III-V compound semiconductors. 7. A superlattice device according to claim 6, wherein (n + m) is equal to an integer. 8. A device according to claim 4, further comprising a third plurality of regions having a third composition and a fourth region.
A fourth plurality of regions having a composition of, wherein the third and fourth plurality of regions are interleaved on the first horizontal superlattice to form a second horizontal superlattice. A superlattice device characterized by: 9. The device of claim 8 wherein the third and fourth compositions are (C) o and (D) p, respectively, and o and p are third and fourth, respectively.
A superlattice device having a ratio of the widths covered by the composition of. 10. A device according to claim 9, wherein C and D are semiconductors. 11. A device according to claim 10, wherein the semiconductor is selected from III-V compound semiconductors. 12. The superlattice device according to claim 9, further comprising a third horizontal superlattice on the horizontal superlattice. 13. The superlattice device according to claim 5, further comprising a first semiconductor layer on the horizontal superlattice. 14. The device according to claim 13, wherein the first plurality of regions has a forbidden band smaller than a forbidden band of adjacent semiconductor regions. 15. The superlattice device according to claim 13, wherein the first plurality of regions are lighter doped than the second plurality of regions. . 16. A device according to claim 13, characterized in that it further comprises first and second electrodes to said device. 17. The device of claim 16 further comprising a third electrode to said device, said third electrode being between said first and second electrodes. Characteristic superlattice device. 18. The device of claim 16, wherein the surface and the semiconductor layer have opposite conductivity types. 19. The device of claim 17, further comprising a second horizontal superlattice consisting of alternating third and fourth regions and a second semiconductor layer. And the second horizontal superlattice and the semiconductor layer are between the first horizontal superlattice and the semiconductor layer, and the second horizontal superlattice is closest to the first semiconductor layer. Characteristic superlattice device. 20. The device of claim 19, wherein the first and third plurality of regions have an opposite conductivity type to the second and fourth plurality of regions. And superlattice device. 21. The device of claim 3, including a semiconductor layer and an active layer between the semiconductor layer and the horizontal superlattice, the surface and the semiconductor layer having opposite conductivity types. A superlattice device, wherein the active layer has a forbidden band smaller than a forbidden band of an adjacent region. 22. The device of claim 21, further comprising first and second electrodes to said device. 23. The superlattice device according to claim 22, wherein the first and second plurality of regions have different refractive indices.